1. Field of Invention
The field of the currently claimed embodiments of this invention relates to methods and compositions for multicompartment and multicomponent, internally structured nanoemulsions.
2. Discussion of Related Art
Long-lived metastable emulsions contain droplets of a dispersed liquid phase dispersed in a continuous liquid phase. In the presence of quiescent thermal excitations, the dispersed droplets do not recombine or coalesce because repulsions between the droplet interfaces are adequate to stabilize thin films of the continuous liquid phase between the droplets.1 Typically, the dispersed phase is highly insoluble in the continuous phase, so coarsening of the average droplet size by Ostwald ripening does not occur.2 However, coalescence of droplets in surfactant- and particle-stabilized emulsions can be induced by a number of means, including raising the osmotic pressure through evaporation of the continuous phase or centrifugation,3 applying electric fields,4,5 applying fluid flow,6,7 changing the surfactant type which affects the hydrophile-lipophile balance (HLB),8 changing pH,9 or adding simple salts.9 In particular, flow-induced coalescence is a physical process in which sufficiently strong fluid flows cause droplets to fuse together, thereby overcoming the stabilizing repulsion between droplet interfaces by causing thinning and rupturing of a film of continuous phase between the droplets.10 Although detailed real-space studies of flow-induced coalescence of pairs of microscale droplets have been made,11 relatively little is known about how flow causes both coalescence and rupturing of droplets in emulsions at high droplet volume fractions beyond the dilute regime and at extremely high strain rates required to produce nanoemulsions. Such extreme flows, which may contain extensional and/or shear components, can cause a number of different complex effects such as droplet deformation, non-equilibrium concentration gradients of surfactant on droplet interfaces, and cavitation, all of which could affect droplet coalescence and rupturing rates.
In classic studies by S. Torza and S. G. Mason,12,13 coalescence between two microscale and larger droplets, each composed of a different immiscible dispersed phase species, has been explored. When a first oil (O1) droplet makes contact with a second different immiscible oil (O2) droplet in water (W), which may contain surfactant, three outcomes are possible; the dominant outcome has the lowest total energy configuration, which can depend on the various liquid-liquid interfacial and line tensions. In the first trivial case, no fusion occurs and the O1 and O2 droplets remain separate. In the second case, the O2 droplet engulfs the O1 droplet, yielding double O1/O2 droplets dispersed in water, also known as an oil-in-oil-in-water O1/O2/W double emulsion. In the third case, O1 and O2 make contact but full engulfing does not occur, producing an anisotropic O1-O2 singlet droplet having a linear morphology. Such a droplet is also called a “Janus” droplet (i.e. after the two-faced deity of gates in Roman mythology). Many linear Janus droplets dispersed in water form a (O1-O2)/W single emulsion.
More recently, emulsions containing microscale and larger Janus droplets have been created using shear and by applying electric fields14 in combination with flows in microfluidic devices. Janus droplets have also been created inside multiple emulsions.15 Just as complex structured dispersions of sphere-packed clusters,16 lithographic colloids,17 and other anisotropic solid particles18,19 are receiving increasing attention, structured liquid droplets, such as Janus droplets, are intriguing as potential building blocks for soft materials and for studying interfacial interactions at small scales. Although nanoemulsions consisting of a single or miscible dispersed phases have been studied,20-23 prior to the approach presented herein, nanoscale linear (O1-O2)/W Janus emulsions, as well as more complex nanoscale variants of compound droplets containing three or more immiscible oils, have not yet been created and studied.
In ionic oil-in-water emulsions, droplets of oil, which have been dispersed in aqueous surfactant solutions by an applied flow, are stabilized against coalescence by adsorbed ionic surfactant molecules on their interfaces. Droplet interfaces in ionic emulsions typically interact through Debye screened-charge repulsions26 that can provide stability against droplet coalescence even at large droplet volume fractions, ϕ, through a short-range repulsive barrier in the potential interaction energy between the droplet interfaces. In stable ionic emulsions, the height of this barrier is typically much larger than thermal energy, kBT, where T is the temperature, when the adsorbed surfactant is present in sufficient quantity. If the oil has negligible solubility in the aqueous surfactant solution, then Ostwald ripening27, 2 is suppressed, and, despite being metastable systems in a technical thermodynamic sense, ionic emulsions can persist as stable dispersions for decades without coarsening or noticeable changes in their droplet size distributions.
One example of a highly persistent emulsion is a poly-(dimethylsiloxane) (PDMS) silicone oil-in-water (O/W) emulsion stabilized using the anionic surfactant sodium dodecyl sulfate (SDS) near or above its critical micelle concentration (CMC) of about 8 mM.28 If the molecular weight of the PDMS is sufficiently large, then its solubility in the SDS solution is extremely low, and Ostwald ripening is effectively suppressed. Likewise, because the Debye-screened repulsive interfacial energy barrier provided by the SDS is far in excess of kBT, thermally driven droplet coalescence is negligible. Both microscale and nanoscale PDMS-SDS O/W emulsions have been used as model systems for many studies, including rheology, light scattering, and optical properties because of their excellent long-term stability.20, 29-31 
For ionic emulsions that do not Ostwald-ripen, it is well known that droplet aggregation and coalescence can be induced by adding salt solutions. In classic experiments, simple salts, such as NaCl, have been added to ionic emulsions to induce interfacial attractions32 and even coalescence between droplets (i.e. “salting out”).33 The presence of the additional dissociated salt ions in the aqueous continuous phase changes the Debye screening length, the potential on the droplet interfaces, and, in some cases, through the common-ion effect, shifts the equilibrium solubility of the surfactant in the solution, thereby altering its Krafft and cloud points.34 Thus, changes in surfactant phase behavior in response to added simple, non-amphiphilic salts are well known to have an important role in emulsion stability.
Apart from emulsions, mixed ionic surfactant systems have been studied extensively, both experimentally and theoretically, in order to understand their complex phase behavior.35-37 Such mixed surfactant systems often exhibit synergistic effects and rich phase behavior, arising from the electrostatic attractions between oppositely charged head groups and hydrophobic tails relative to entropic forces. In particular, anionic and cationic surfactant mixtures can cause an increase in surface activity beyond what might be expected based on their individual behavior38, 39 and double-layer effects can appear to be negligible.40 Additionally, these mixed systems can form phases composed of molecular dispersions, micelles, vesicles, crystal precipitates, or combinations thereof, depending on the relative concentrations of surfactants and water.41, 42 Anionic and cationic lipid mixtures can even be used to produce liposomes.43 
Multi-compartment O/W nanoemulsions, such as Janus and Cerberus nanodroplets, have been produced in a massively parallel process through extreme flow-induced fusion.44 However, in the absence of strong flow, a surfactant-induced interfacial destabilization process has not yet been used to produce such complex nanoscale droplet structures. Although multi-component compound droplet or particle structures have been created at the microscale, particularly using microfluidics in a serial process,45-47 three-immiscible-component and higher-order compound droplets have not yet been produced via a massively parallel process at the nanoscale. Moreover, experimental evidence that could provide insight into potential mechanisms causing droplet coalescence and enabling control of droplet fusion in ionic emulsions, which have been destabilized by adding solutions of surfactant salts, is lacking. Thus, studying the behavior of ionic emulsions that are mixed with oppositely charged surfactant solutions, as well as mixtures of two emulsions stabilized by oppositely charged surfactants, would provide a better understanding of interfacial destabilization mechanisms that lead to droplet coalescence and could even provide control over that process.